1. Field of the Disclosure
The present disclosure is related to estimating bulk shale volume in an earth formation during drilling of the formation by processing in situ measurements obtained downhole.
2. Description of the Related Art
In petroleum exploration, various parameters of earth formation are measured to estimate a presence of oil. One useful parameter is bulk shale volume (BSV), which is related to the amount of shale in the earth formation. Bulk shale volume estimates are generally useful for real-time pore pressure prediction, real-time petrophysical analysis and for real-time rock mechanical issues. As a result, real-time processing of measurements related to BSV is desirable.
Various methods are in use for obtaining estimates of bulk shale volume. In wireline testing, for example, a wireline conveys various measurement sensors into a wellbore to obtain measurements related to BSV. In such operations, large amounts of data are typically acquired and later transported to a surface location and downloaded to a surface processor for analysis. Although analysis at the surface processor yields a reasonable estimate of the desired parameter, due to the need to transport the data to the surface for calculations, real-time estimation is not possible. In another method known as Logging-While-Drilling (LWD), sensors are conveyed into the wellbore on a bottomhole assembly (BHA) of a drill string along with a drilling apparatus. Data can be stored in a memory downhole and later dumped to a surface processor for calculations as in wireline testing. In general, however, since data is acquired continuously during the drilling operation, it is desirable to perform relevant calculations downhole.
Several issues concerning estimating bulk shale volume in LWD operations are well-known. Representative measurement values of shale and sand are generally need to obtain an estimate of bulk shale volume. Ideally, both sand and shale would be encountered upon pentration of the formation at the beginning of drilling and would thereby give immediate initial estimates usable in ensuing BSV calculations. However, this is seldom the case. In general, at the start of drilling, the drill string may at first penetrate only shale or only sands, rendering it difficult to obtain an initial estimate of the percentage of bulk shale in the formation. Also, the amount of data obtained typically depends on the amount of the time the sensor is in the wellbore and the rate of penetration of the drill string. Thus, first estimates tend to suffer due to the small amount of data initially available. There is therefore a need for a method of providing a quick estimate of bulk shale volume in real-time during logging while drilling operations.